Hitherto, an optical disc for carrying out recording/reproduction of information by irradiation of laser beams, magneto-optical discs, various write-once type optical discs which are, digital audio discs so called compact discs, and optical video discs, etc. have been put into practical use.
The compact disc and/or the optical video disc among these optical discs are reproduction only optical discs, are ordinarily of a structure in which a reflection layer is formed on a transparent base (substrate) where data pits corresponding to information signals are formed in a recessed form.
As the transparent base, because cost can beheld down to a lower level particularly in the case where those bases are mass-produced, disc-shaped bases obtained by injection-molding resin such as polycarbonate, etc. are frequently used. Data pits are arranged and formed as a concave recessed form on tracks set in concentrical form or in spiral form of the transparent base.
Moreover, the reflection layer is stacked on the surface where data pits are formed of the transparent base. Aluminum reflection film is generally used because of its high reflection factor and good thermal conductivity.
In the reproduction only optical disc of such a configuration, a technique is employed to detect a difference between quantity of reflected light at the pit portion and that at the portion where no pit exist, i.e., the mirror portion when laser beams are irradiated from the transparent base side to reproduce the pit pattern on the tracks on the basis of the difference. In this case, in order to carry out precise signal reproduction free from error by such a technique, it is necessary that a laser beam spot is precisely irradiated onto the track where a pit pattern to be read out is formed. For this reason, in the optical disc drive, tracking servo of the optical pick-up is carried out.
As the optical disc system for scanning, by laser beams, tracks formed in a concentric form or in a spiral form to carry out recording/reproduction of various data, there are known the optical disc system of the CLV system for rotationally driving the optical disc at a constant linear velocity (CLV) to carry out recording/reproduction of data, and the optical disc system of the CAV system for rotationally driving the optical disc at a constant angular velocity (CAV) to carry out recording/reproduction of data.
Further, there are known, in terms of the servo system, optical discs of the continuous servo system to carry out tracking control, etc. by using pre-grooves continuously provided along the tracks, and optical discs of the sample servo system to carry out tracking control, etc. by making use of servo areas discretely provided on the tracks.
In the case of the optical discs of the sample servo system, respective tracks are divided into plural data areas, and two servo pits are respectively formed in the servo areas between the respective adjacent two data areas.
Respective two servo pits are formed as so called wobble pits by a geometrical concavity at positions respectively spaced by a 1/4 track width in a radial direction from a center line of the track by arrangement in wobble form in such a manner that the scanning line of the laser beam spot, i.e., the center line of the track is put therebetween before and after.
Tracking servo of the laser spot is carried out on the basis of servo pits arranged in a wobble form. Such servo pits are used for tracking servo as stated above, and are also utilized for generating a clock signal for the reproducing system. Accordingly, in the sample servo system, shape and/or position accuracy of the servo pit are very important for satisfactorily constituting the reproducing system.
Meanwhile, as the transparent base (substrate) of the optical disc, injection-molded bases are frequently used as described above. Such injection-molded base is made up, e.g., in a manner as described below.
Namely, a stamper where the inversion pattern of data pits and servo pits is formed is fixed within a metal mold to carry out mold-clamping to inject resin such as molten polycarbonate, etc. into the metal mold in which the stamper is fixed. Then, pressure is applied to the metal mold while cooling it. After the resin is solidified, the metal mold is opened to peel (separate) the resin from the stamper. Thus, an injection-molded base for an optical disc on which the inversion pattern of the stamper has been transferred is obtained.
In this case, transfer failure (or unsatisfactory transfer) of pits resulting from thermal contraction of the base is the problem.
Namely, the base comprised of resin has great coefficient of thermal expansion, so thermal contraction takes place in the cooling process of the injection molding and the peeling (separation) process from the stamper. As a result, multi-transfer (multiple transfer) of pits and/or deformation of pit shape would take place.
For example, as shown in FIG. 1, a base 1 comprised of resin attempts to undergo thermal contraction in the cooling process. At this time, because of the effect for preventing the flow of resin by pressure given by the metal mold or inversion pits 3 formed at a stamper 2 (which will be called anchor effect hereinafter), the resin is resultantly in the sticking state without moving on the stamper 2. However, when attempt is made to peel (separate) the base 1 from the stamper 2, its stress F is released. As a result, the base 1 contracts. In this instance, pit shape is deformed, from difference between contraction quantity at the sticking portion of the base 1 and that at the peeled (separated) portion thereof, at the boundary between the portion sticking on the stamper 2 and the peeled (separated) portion as shown in FIG. 2. As a result, transfer failure (or unsatisfactory transfer) would take place. Such pit deformation is apt to take place at pits adjacent to the long mirror portion in which the anchor effect by the inversion pits is difficult to obtain, e.g., at servo pits. Namely, the anchor effect by the inversion pits is difficult to obtain at the portion where the mirror portion is continued over a relatively longer distance such as the portion between the servo pit and the data pit. As a result, a portion of the base, i.e., resin moves on the stamper 2 by thermal contraction. Followed by this, the pit shape is deformed and/or pits are caused to undergo multi-transfer. Image by the AFM (Atomic Force Microscopy) indicating the state of deformation of pits is shown in FIG. 3. As is clear from FIG. 3, pits would be formed so as to take a trapezoidal shape.
As described above, because servo pits are used for tracking servo and are also used as a clock signal for the reproducing system, if their shapes are not correct, not only deviation in tracking takes place, but also deviation in the timing for generation of the clock signal takes place, with the result that bad influence is exerted on the reproducing system.
This invention has been proposed in view of such actual circumstances of the prior art, and its object is to provide an optical disc and an optical disc system in which various pits including servo pits can be formed in a satisfactory form, and tracking servo and/or generation of clock are precisely carried out.